Generally during the operations of electronic apparatuses, there exist various power source noises and clock pulse source noises. Such noises are propagated through circuit power lines or signal lines across circuits, with the result that malfunctions of the electronic apparatus are liable to occur.
Further, the power source noises and the clock pulse noises which are generated from the interior of an electronic apparatus can propagate to other electronic apparatuses through the power lines to cause malfunctions in the other electronic apparatuses. In the same way, noises can be introduced from another electronic apparatus to cause a malfunction. In generic terms, these are called electro-magnetic interferences.
In order to prevent such electro-magnetic interferences, noise filters are disposed between circuits and a circuit power source, and between circuits and a clock pulse source. Such a noise filter forms a 3-terminal LC filter and includes two inductors and one capacitor.
FIG. 1 is an equivalent circuit for a 3-terminal LC noise filter.
Referring to FIG. 1, The noise signal component which is inputted through terminals 10 and 11 is bypassed to the ground by a first LC filter which consists of a first inductor L1 and a capacitor C1. Meanwhile, the noise signal component which is inputted through terminal 12 and terminal 11 is bypassed to the ground by a second LC filter which consists of a second inductor L2 and the capacitor C1.
The first and second inductors L1 and L2 are designed in such a manner that they should have the same inductance value, and therefore, the first and second LC filters have the same filtering characteristics.
FIG. 2 is a perspective view showing the structure of a conventional 3-terminal noise filter.
Referring to this drawing, a long lead wire 13 is bent twice rectangularly, and first and second inductor leads 14 and 15 corresponding to terminals 10 and 12 of the equivalent circuit of FIG. 1 are formed. Cylindrical ferrite beads 19 and 20 are fitted respectively to the first and second inductor leads 14 and 15 up to the bent portions.
Then a first electrode 22 of a chip capacitor 21 is soldered to the middle portion of an upper bar 16 which connects the first and second inductor leads 14 and 15 to each other. Then a second electrode 23 of the chip capacitor 21 is soldered to a capacitor lead 17 which is disposed between the first and second inductor leads 14 and 15 in parallel with the first and second inductor leads 14 and 15, thereby completing the final structure of the 3-terminal LC noise filter.
Here, a head portion 18 of the capacitor lead 17 which is connected to the second electrode 23 of the chip capacitor 21 is shaped like the head of a nail to increase the contact area.
However, in such a conventional 3-terminal noise filter as described above, when the chip capacitor 21 is soldered between the center of the upper portion 16 and the capacitor lead 17, much time is consumed and care has to be exercised for the position setting of the chip capacitor 21. Consequently, work efficiency and productivity are lowered, as well as producing large amounts of soldering defects.
Particularly, when the 3-terminal noise filter is used for radio frequency operation, there exists a length difference 1 between the capacitor lead 17 and the first and second inductor leads 14 and 15. When such a 3-terminal noise filter is positioned on a circuit board, the filtering characteristics are varied due to the inductance component of the length difference 1.
FIG. 3 illustrates another conventional 3-terminal noise filter.
Referring to FIG. 3, there is used a lead frame plate instead of the wire of FIG. 2. First a metal plate is cut rectangularly to form a lead frame 26 having first and second inductor leads 24 and 25. Then the first and second inductor leads 24 and 25 are inserted into first and second ferrite beads 19 and 20. Then first and second electrodes 22 and 23 of the chip capacitor 21 are soldered between a capacitor lead 27 (shaped like the lead frame) and the center of an upper portion which connects the first and second inductor leads 24 and 25, thereby completing the final structure of the 3-terminal noise filter.
This structure has advantages in ease of soldering and improvement of yield, but the lead frame is very expensive, with the result that the manufacturing cost is increased.